1. Field of the Invention
This invention relates to flash-spinning of polymeric plexifilamentary film-fibril strand. More particularly, the invention concerns an improved process in which the strand is flash-spun from mixtures of methylene chloride and a co-solvent.
2. Description of the Prior Art
Blades and White, U.S. Pat. No. 3,081,519, describes a flash-spinning process for producing plexifilamentary film-fibril strands from fiber-forming polymers. A solution of the polymer in a liquid, which is a non-solvent for the polymer at or below its normal boiling point, is extruded at a temperature above the normal boiling point of the liquid and at autogenous or higher pressure into a medium of lower temperature and substantially lower pressure. This flash spinning causes the liquid to vaporize and thereby cool the plexifilamentary film-fibril strand that forms from the polymer. Preferred polymers include crystalline polyhydrocarbons such as polyethylene and polypropylene.
According to U.S. Pat. No. 3,081,519 the following liquids are useful in the flash-spinning process: aromatic hydrocarbons such as benzene, toluene, etc.; aliphatic hydrocarbons such as butane, pentane, hexane, heptane, octane, and their isomers and homologs; alicyclic hydrocarbons such as cyclohexane; unsaturated hydrocarbons; halogenated hydrocarbons such as methylene chloride, carbon tetrachloride, chloroform, ethyl chloride, methyl chloride; alcohols; esters; ethers; ketones; nitriles; amides; fluorocarbons; sulfur dioxide; carbon disulfide; nitromethane; water; and mixtures of the above liquids. The patent further states that the flash-spinning solution additionally may contain a dissolved gas, such as nitrogen, carbon dioxide, helium, hydrogen, methane, propane, butane, ethylene, propylene, butane, etc. Preferred for improving plexifilament fibrillation are the less soluble gases, i.e., those that dissolve to a less than 7% concentration in the polymer solution under the spinning conditions.
Many examples of U.S. Pat. No. 3,018,519 and British Patents 891,943 and 891,945 describe flash-spinning of polyethylene from methylene chloride or from methylene chloride with a co-solvent. However, the resultant products are generally unsatisfactory for producing plexifilamentary film-fibril strands of the quality required for commercial production of spunbonded sheet products. Commercial spunbonded products made from polyethylene plexifilamentary film-fibril strands have been successfully produced with the polyethylene being flash-spun from trichlorofluoromethane (Freon-11). Although Freon-11 has been used extensively for this purpose, the escape of such a halocarbon into the atmosphere has been implicated as a serious source of depletion of the earth's ozone. A general discussion of the ozone-depletion problem is presented, for example, by P. S. Zurer, "Search Intensifies for Alternatives to Ozone-Depleting Halocarbons", Chemical & Engineering News, pages 17-20 (Feb. 8, 1988). The substitution of methylene chloride for trichlorofluoromethane in the commercial flash-spinning process should avoid the ozone depletion problem, but plexifilamentary film-fibril strands of polyethylene which are flash-spun from methylene chloride, with or without co-solvent, as exemplified in the referred-to patents, are inadequate; they do not meet the high fibrillation quality of the strands produced by the commercial process which employs trichlorofluoromethane as the spin solvent.
An object of this invention is to provide an improved process for flash-spinning polyethylene plexifilamentary film-fibril strand of high quality from a fluid that should not present ozone-depletion hazards,